In physical cosmology and astronomy, dark energy is an unknown form of
energy which is hypothesized to permeate all of space, tending to
accelerate the expansion of the universe.[1][2]
Dark energyDark energy is the
most accepted hypothesis to explain the observations since the 1990s
indicating that the universe is expanding at an accelerating rate.
Assuming that the standard model of cosmology is correct, the best
current measurements indicate that dark energy contributes 68.3% of
the total energy in the present-day observable universe. The
mass–energy of dark matter and ordinary (baryonic) matter contribute
26.8% and 4.9%, respectively, and other components such as neutrinos
and photons contribute a very small amount.[3][4][5][6] The density of
dark energy (~ 7 × 10−30 g/cm3) is very low,
much less than the density of ordinary matter or dark matter within
galaxies. However, it dominates the mass–energy of the universe
because it is uniform across space.[7][8][9]
Two proposed forms for dark energy are the cosmological
constant,[10][11] representing a constant energy density filling space
homogeneously, and scalar fields such as quintessence or moduli,
dynamic quantities whose energy density can vary in time and space.
Contributions from scalar fields that are constant in space are
usually also included in the cosmological constant. The cosmological
constant can be formulated to be equivalent to the zero-point
radiation of space i.e. the vacuum energy.[12] Scalar fields that
change in space can be difficult to distinguish from a cosmological
constant because the change may be extremely slow.

5 Implications for the fate of the universe
6 In philosophy of science
7 See also
8 Notes
9 References
10 External links

History of discovery and previous speculation[edit]
Einstein's cosmological constant[edit]
The "cosmological constant" is a constant term that can be added to
Einstein's field equationEinstein's field equation of General Relativity. If considered as a
"source term" in the field equation, it can be viewed as equivalent to
the mass of empty space (which conceptually could be either positive
or negative), or "vacuum energy".
The cosmological constant was first proposed by Einstein as a
mechanism to obtain a solution of the gravitational field equation
that would lead to a static universe, effectively using dark energy to
balance gravity.[13] Einstein gave the cosmological constant the
symbol Λ (capital lambda).
The mechanism was an example of fine-tuning, and it was later realized
that Einstein's static universe would not be stable: local
inhomogeneities would ultimately lead to either the runaway expansion
or contraction of the universe. The equilibrium is unstable: if the
universe expands slightly, then the expansion releases vacuum energy,
which causes yet more expansion. Likewise, a universe which contracts
slightly will continue contracting. These sorts of disturbances are
inevitable, due to the uneven distribution of matter throughout the
universe. Further, observations made by
Edwin HubbleEdwin Hubble in 1929 showed
that the universe appears to be expanding and not static at all.
Einstein reportedly referred to his failure to predict the idea of a
dynamic universe, in contrast to a static universe, as his greatest
blunder.[14]
Inflationary dark energy[edit]
Alan GuthAlan Guth and
Alexei StarobinskyAlexei Starobinsky proposed in 1980 that a negative
pressure field, similar in concept to dark energy, could drive cosmic
inflation in the very early universe. Inflation postulates that some
repulsive force, qualitatively similar to dark energy, resulted in an
enormous and exponential expansion of the universe slightly after the
Big Bang. Such expansion is an essential feature of most current
models of the Big Bang. However, inflation must have occurred at a
much higher energy density than the dark energy we observe today and
is thought to have completely ended when the universe was just a
fraction of a second old. It is unclear what relation, if any, exists
between dark energy and inflation. Even after inflationary models
became accepted, the cosmological constant was thought to be
irrelevant to the current universe.
Nearly all inflation models predict that the total (matter+energy)
density of the universe should be very close to the critical density.
During the 1980s, most cosmological research focused on models with
critical density in matter only, usually 95% cold dark matter and 5%
ordinary matter (baryons). These models were found to be successful at
forming realistic galaxies and clusters, but some problems appeared in
the late 1980s: in particular, the model required a value for the
Hubble constantHubble constant lower than preferred by observations, and the model
under-predicted observations of large-scale galaxy clustering. These
difficulties became stronger after the discovery of anisotropy in the
cosmic microwave background by the COBE spacecraft in 1992, and
several modified CDM models came under active study through the
mid-1990s: these included the
Lambda-CDM modelLambda-CDM model and a mixed cold/hot
dark matter model. The first direct evidence for dark energy came from
supernova observations in 1998 of accelerated expansion in Riess et
al.[15] and in Perlmutter et al.,[16] and the
Lambda-CDM modelLambda-CDM model then
became the leading model. Soon after, dark energy was supported by
independent observations: in 2000, the BOOMERanG and Maxima cosmic
microwave background experiments observed the first acoustic peak in
the CMB, showing that the total (matter+energy) density is close to
100% of critical density. Then in 2001, the 2dF
GalaxyGalaxyRedshiftRedshift Survey
gave strong evidence that the matter density is around 30% of
critical. The large difference between these two supports a smooth
component of dark energy making up the difference. Much more precise
measurements from
WMAPWMAP in 2003–2010 have continued to support the
standard model and give more accurate measurements of the key
parameters.
The term "dark energy", echoing Fritz Zwicky's "dark matter" from the
1930s, was coined by Michael Turner in 1998.[17]
Change in expansion over time[edit]

Diagram representing the accelerated expansion of the universe due to
dark energy.

High-precision measurements of the expansion of the universe are
required to understand how the expansion rate changes over time and
space. In general relativity, the evolution of the expansion rate is
estimated from the curvature of the universe and the cosmological
equation of state (the relationship between temperature, pressure, and
combined matter, energy, and vacuum energy density for any region of
space). Measuring the equation of state for dark energy is one of the
biggest efforts in observational cosmology today. Adding the
cosmological constant to cosmology's standard FLRW metric leads to the
Lambda-CDM model, which has been referred to as the "standard model of
cosmology" because of its precise agreement with observations.
As of 2013, the
Lambda-CDM modelLambda-CDM model is consistent with a series of
increasingly rigorous cosmological observations, including the Planck
spacecraft and the
SupernovaSupernova Legacy Survey. First results from the
SNLS reveal that the average behavior (i.e., equation of state) of
dark energy behaves like Einstein's cosmological constant to a
precision of 10%.[18] Recent results from the Hubble Space Telescope
Higher-Z Team indicate that dark energy has been present for at least
9 billion years and during the period preceding cosmic acceleration.
Nature[edit]

Axis scale: billion years
Also see:
HumanHuman timeline and Life timeline

The nature of dark energy is more hypothetical than that of dark
matter, and many things about the nature of dark energy remain matters
of speculation.[19]
Dark energyDark energy is thought to be very homogeneous, not
very dense and is not known to interact through any of the fundamental
forces other than gravity. Since it is quite rarefied, un-massive —
roughly 10−27 kg/m3 — it is unlikely to be detectable in
laboratory experiments. The reason dark energy can have such a
profound effect on the universe, making up 68% of universal density,
in spite of being so rarefied is because it uniformly fills otherwise
empty space.
Independently of its actual nature, dark energy would need to have a
strong negative pressure (acting repulsively) like radiation pressure
in a metamaterial[20] to explain the observed acceleration of the
expansion of the universe. According to general relativity, the
pressure within a substance contributes to its gravitational
attraction for other things just as its mass density does. This
happens because the physical quantity that causes matter to generate
gravitational effects is the stress–energy tensor, which contains
both the energy (or matter) density of a substance and its pressure
and viscosity[dubious – discuss]. In the
Friedmann–Lemaître–Robertson–Walker metric, it can be shown
that a strong constant negative pressure in all the universe causes an
acceleration in universe expansion if the universe is already
expanding, or a deceleration in universe contraction if the universe
is already contracting. This accelerating expansion effect is
sometimes labeled "gravitational repulsion".
Technical definition[edit]
See also: Friedmann equations
In standard cosmology, there are three components of the universe:
matter, radiation and dark energy.
MatterMatter is anything whose energy
density scales with the inverse cube of the scale factor, i.e. ρ ∝
a−3, while radiation is anything which scales to the inverse fourth
power of the scale factor ρ ∝ a−4. This can be understood
intuitively: for an ordinary particle in a square box, doubling the
length of a side of the box decreases the density (and hence energy
density) by a factor of eight (23). For radiation, the decrease in
energy density is greater, because an increase in spatial distance
also causes a redshift.[21]
The final component, dark energy, is an intrinsic property of space,
and so has a constant energy density regardless of the volume under
consideration (ρ ∝ a0). Thus, unlike ordinary matter, it does not
get diluted with the expansion of space, leading to an accelerated
expansion, instead of the deceleration expected from a universe
containing only ordinary matter.
Evidence of existence[edit]
The evidence for dark energy is indirect but comes from three
independent sources:

Distance measurements and their relation to redshift, which suggest
the universe has expanded more in the last half of its life.[22]
The theoretical need for a type of additional energy that is not
matter or dark matter to form the observationally flat universe
(absence of any detectable global curvature).
It can be inferred from measures of large scale wave-patterns of mass
density in the universe.

In 1998, the High-Z
SupernovaSupernova Search Team[15] published observations
of Type Ia ("one-A") supernovae. In 1999, the
SupernovaSupernova Cosmology
Project[16] followed by suggesting that the expansion of the universe
is accelerating.[23] The 2011 Nobel Prize in Physics was awarded to
Saul Perlmutter,
Brian P. SchmidtBrian P. Schmidt and
Adam G. RiessAdam G. Riess for their
leadership in the discovery.[24][25]
Since then, these observations have been corroborated by several
independent sources. Measurements of the cosmic microwave background,
gravitational lensing, and the large-scale structure of the cosmos as
well as improved measurements of supernovae have been consistent with
the Lambda-CDM model.[26] Some people argue that the only indications
for the existence of dark energy are observations of distance
measurements and the associated redshifts. Cosmic microwave background
anisotropies and baryon acoustic oscillations only serve to
demonstrate that distances to a given redshift are larger than would
be expected from a "dusty" Friedmann–Lemaître universe and the
local measured Hubble constant.[27]
Supernovae are useful for cosmology because they are excellent
standard candles across cosmological distances. They allow the
expansion history of the universe to be measured by looking at the
relationship between the distance to an object and its redshift, which
gives how fast it is receding from us. The relationship is roughly
linear, according to Hubble's law. It is relatively easy to measure
redshift, but finding the distance to an object is more difficult.
Usually, astronomers use standard candles: objects for which the
intrinsic brightness, the absolute magnitude, is known. This allows
the object's distance to be measured from its actual observed
brightness, or apparent magnitude. Type Ia supernovae are the
best-known standard candles across cosmological distances because of
their extreme and consistent luminosity.
Recent observations of supernovae are consistent with a universe made
up 71.3% of dark energy and 27.4% of a combination of dark matter and
baryonic matter.[28]
Cosmic microwave background[edit]

Estimated division of total energy in the universe into matter, dark
matter and dark energy based on five years of
WMAPWMAP data.[29]

The existence of dark energy, in whatever form, is needed to reconcile
the measured geometry of space with the total amount of matter in the
universe. Measurements of cosmic microwave background (CMB)
anisotropies indicate that the universe is close to flat. For the
shape of the universe to be flat, the mass/energy density of the
universe must be equal to the critical density. The total amount of
matter in the universe (including baryons and dark matter), as
measured from the CMB spectrum, accounts for only about 30% of the
critical density. This implies the existence of an additional form of
energy to account for the remaining 70%.[26] The Wilkinson Microwave
AnisotropyAnisotropy Probe (WMAP) spacecraft seven-year analysis estimated a
universe made up of 72.8% dark energy, 22.7% dark matter and 4.5%
ordinary matter.[5] Work done in 2013 based on the Planck spacecraft
observations of the CMB gave a more accurate estimate of 68.3% of dark
energy, 26.8% of dark matter and 4.9% of ordinary matter.[30]
Large-scale structure[edit]
The theory of large-scale structure, which governs the formation of
structures in the universe (stars, quasars, galaxies and galaxy groups
and clusters), also suggests that the density of matter in the
universe is only 30% of the critical density.
A 2011 survey, the WiggleZ galaxy survey of more than 200,000
galaxies, provided further evidence towards the existence of dark
energy, although the exact physics behind it remains unknown.[31][32]
The WiggleZ survey from the Australian Astronomical Observatory
scanned the galaxies to determine their redshift. Then, by exploiting
the fact that baryon acoustic oscillations have left voids regularly
of ~150 Mpc diameter, surrounded by the galaxies, the voids were used
as standard rulers to estimate distances to galaxies as far as 2,000
Mpc (redshift 0.6), allowing for accurate estimate of the speeds of
galaxies from their redshift and distance. The data confirmed cosmic
acceleration up to half of the age of the universe (7 billion years)
and constrain its inhomogeneity to 1 part in 10.[32] This provides a
confirmation to cosmic acceleration independent of supernovae.
Late-time integrated Sachs-Wolfe effect[edit]
Accelerated cosmic expansion causes gravitational potential wells and
hills to flatten as photons pass through them, producing cold spots
and hot spots on the CMB aligned with vast supervoids and
superclusters. This so-called late-time Integrated Sachs–Wolfe
effect (ISW) is a direct signal of dark energy in a flat universe.[33]
It was reported at high significance in 2008 by Ho et al.[34] and
Giannantonio et al.[35]
Observational
Hubble constantHubble constant data[edit]
A new approach to test evidence of dark energy through observational
Hubble constantHubble constant data (OHD) has gained significant attention in recent
years.[36][37][38][39] The Hubble constant, H(z), is measured as a
function of cosmological redshift. OHD directly tracks the expansion
history of the universe by taking passively evolving early-type
galaxies as “cosmic chronometers”.[40] From this point, this
approach provides standard clocks in the universe. The core of this
idea is the measurement of the differential age evolution as a
function of redshift of these cosmic chronometers. Thus, it provides a
direct estimate of the Hubble parameter

The reliance on a differential quantity, Δz/Δt, can minimize many
common issues and systematic effects; and as a direct measurement of
the Hubble parameter instead of its integral, like supernovae and
baryon acoustic oscillations (BAO), it brings more information and is
appealing in computation. For these reasons, it has been widely used
to examine the accelerated cosmic expansion and study properties of
dark energy.
Theories of dark energy[edit]
Dark energy's status as a hypothetical force with unknown properties
makes it a very active target of research. The problem is attacked
from a great variety of angles, such as modifying the prevailing
theory of gravity (general relativity), attempting to pin down the
properties of dark energy, and finding alternative ways to explain the
observational data.

The simplest explanation for dark energy is that it is an intrinsic,
fundamental energy of space. This is the cosmological constant,
usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM
model). Since energy and mass are related according to the equation E
= mc2, Einstein's theory of general relativity predicts that this
energy will have a gravitational effect. It is sometimes called a
vacuum energy because it is the energy density of empty vacuum.
The cosmological constant has negative pressure equal to its energy
density and so causes the expansion of the universe to accelerate. The
reason a cosmological constant has negative pressure can be seen from
classical thermodynamics. In general, energy must be lost from inside
a container (the container must do work on its environment) in order
for the volume to increase. Specifically, a change in volume dV
requires work done equal to a change of energy −P dV, where P
is the pressure. But the amount of energy in a container full of
vacuum actually increases when the volume increases, because the
energy is equal to ρV, where ρ is the energy density of the
cosmological constant. Therefore, P is negative and, in fact,
P = −ρ.
There are two major advantages for the cosmological constant. The
first is that it is simple. Einstein had in fact introduced this term
in his original formulation of general relativity such as to get a
static universe. Although he later discarded the term after Hubble
found that the universe is expanding, a nonzero cosmological constant
can act as dark energy, without otherwise changing the Einstein field
equations. The other advantage is that there is a natural explanation
for its origin. Most quantum field theories predict vacuum
fluctuations that would give the vacuum this sort of energy. This is
related to the Casimir effect, in which there is a small suction into
regions where virtual particles are geometrically inhibited from
forming (e.g. between plates with tiny separation).
A major outstanding problem is that the same quantum field theories
predict a huge cosmological constant, more than 100 orders of
magnitude too large.[11] This would need to be almost, but not
exactly, cancelled by an equally large term of the opposite sign. Some
supersymmetric theories require a cosmological constant that is
exactly zero,[43] which does not help because supersymmetry must be
broken.
Nonetheless, the cosmological constant is the most economical solution
to the problem of cosmic acceleration. Thus, the current standard
model of cosmology, the Lambda-CDM model, includes the cosmological
constant as an essential feature.
Modified gravity[edit]
The evidence for dark energy is heavily dependent on the theory of
general relativity. Therefore, it is conceivable that a modification
to general relativity also eliminates the need for dark energy. There
are very many such theories, and research is ongoing.[44][45] The
measurement of the speed of gravity with the gravitational wave event
GW170817GW170817 ruled out many modified gravity theories as alternative
explanation to dark energy.[46][47][48]
Quintessence[edit]
Main article:
Quintessence (physics)
In quintessence models of dark energy, the observed acceleration of
the scale factor is caused by the potential energy of a dynamical
field, referred to as quintessence field.
Quintessence differs from
the cosmological constant in that it can vary in space and time. In
order for it not to clump and form structure like matter, the field
must be very light so that it has a large Compton wavelength.
No evidence of quintessence is yet available, but it has not been
ruled out either. It generally predicts a slightly slower acceleration
of the expansion of the universe than the cosmological constant. Some
scientists think that the best evidence for quintessence would come
from violations of Einstein's equivalence principle and variation of
the fundamental constants in space or time.[49] Scalar fields are
predicted by the
Standard ModelStandard Model of particle physics and string theory,
but an analogous problem to the cosmological constant problem (or the
problem of constructing models of cosmological inflation) occurs:
renormalization theory predicts that scalar fields should acquire
large masses.
The coincidence problem asks why the acceleration of the Universe
began when it did. If acceleration began earlier in the universe,
structures such as galaxies would never have had time to form, and
life, at least as we know it, would never have had a chance to exist.
Proponents of the anthropic principle view this as support for their
arguments. However, many models of quintessence have a so-called
"tracker" behavior, which solves this problem. In these models, the
quintessence field has a density which closely tracks (but is less
than) the radiation density until matter-radiation equality, which
triggers quintessence to start behaving as dark energy, eventually
dominating the universe. This naturally sets the low energy scale of
the dark energy.[50][51]
In 2004, when scientists fit the evolution of dark energy with the
cosmological data, they found that the equation of state had possibly
crossed the cosmological constant boundary (w = −1) from
above to below. A No-Go theorem has been proved that gives this
scenario at least two degrees of freedom as required for dark energy
models. This scenario is so-called Quintom scenario.
Some special cases of quintessence are phantom energy, in which the
energy density of quintessence actually increases with time, and
k-essence (short for kinetic quintessence) which has a non-standard
form of kinetic energy such as a negative kinetic energy.[52] They can
have unusual properties: phantom energy, for example, can cause a Big
Rip.
Interacting dark energy[edit]
This class of theories attempts to come up with an all-encompassing
theory of both dark matter and dark energy as a single phenomenon that
modifies the laws of gravity at various scales. This could for example
treat dark energy and dark matter as different facets of the same
unknown substance,[53] or postulate that cold dark matter decays into
dark energy.[54] Another class of theories that unifies dark matter
and dark energy are suggested to be covariant theories of modified
gravities. These theories alter the dynamics of the space-time such
that the modified dynamic stems what have been assigned to the
presence of dark energy and dark matter.[55]
Variable dark energy models[edit]
The density of dark energy might have varied in time over the history
of the universe. Modern observational data allow for estimates of the
present density. Using baryon acoustic oscillations, it is possible to
investigate the effect of dark energy in the history of the Universe,
and constrain parameters of the equation of state of dark energy. To
that end, several models have been proposed. One of the most popular
models is the Chevallier–Polarski–Linder model (CPL).[56][57] Some
other common models are, (Barboza & Alcaniz. 2008),[58] (Jassal et
al. 2005),[59] (Wetterich. 2004).[60]
Observational skepticism[edit]
Some alternatives to dark energy aim to explain the observational data
by a more refined use of established theories. In this scenario, dark
energy doesn't actually exist, and is merely a measurement artifact.
For example, if we are located in an emptier-than-average region of
space, the observed cosmic expansion rate could be mistaken for a
variation in time, or acceleration.[61][62][63][64] A different
approach uses a cosmological extension of the equivalence principle to
show how space might appear to be expanding more rapidly in the voids
surrounding our local cluster. While weak, such effects considered
cumulatively over billions of years could become significant, creating
the illusion of cosmic acceleration, and making it appear as if we
live in a Hubble bubble.[65][66][67] Yet other possibilities are that
the accelerated expansion of the universe is an illusion caused by the
relative motion of us to the rest of the universe,[68][69] or that the
supernovae sample size used wasn't large enough.[70][71]
Implications for the fate of the universe[edit]
Cosmologists estimate that the acceleration began roughly 5 billion
years ago.[72][notes 1] Before that, it is thought that the expansion
was decelerating, due to the attractive influence of matter. The
density of dark matter in an expanding universe decreases more quickly
than dark energy, and eventually the dark energy dominates.
Specifically, when the volume of the universe doubles, the density of
dark matter is halved, but the density of dark energy is nearly
unchanged (it is exactly constant in the case of a cosmological
constant).
Projections into the future can differ radically for different models
of dark energy. For a cosmological constant, or any other model that
predicts that the acceleration will continue indefinitely, the
ultimate result will be that galaxies outside the
Local GroupLocal Group will
have a line-of-sight velocity that continually increases with time,
eventually far exceeding the speed of light.[73] This is not a
violation of special relativity because the notion of "velocity" used
here is different from that of velocity in a local inertial frame of
reference, which is still constrained to be less than the speed of
light for any massive object (see Uses of the proper distance for a
discussion of the subtleties of defining any notion of relative
velocity in cosmology). Because the Hubble parameter is decreasing
with time, there can actually be cases where a galaxy that is receding
from us faster than light does manage to emit a signal which reaches
us eventually.[74][75] However, because of the accelerating expansion,
it is projected that most galaxies will eventually cross a type of
cosmological event horizon where any light they emit past that point
will never be able to reach us at any time in the infinite future[76]
because the light never reaches a point where its "peculiar velocity"
toward us exceeds the expansion velocity away from us (these two
notions of velocity are also discussed in Uses of the proper
distance). Assuming the dark energy is constant (a cosmological
constant), the current distance to this cosmological event horizon is
about 16 billion light years, meaning that a signal from an event
happening at present would eventually be able to reach us in the
future if the event were less than 16 billion light years away, but
the signal would never reach us if the event were more than 16 billion
light years away.[75]
As galaxies approach the point of crossing this cosmological event
horizon, the light from them will become more and more redshifted, to
the point where the wavelength becomes too large to detect in practice
and the galaxies appear to vanish completely[77][78] (see Future of an
expanding universe). Planet Earth, the Milky Way, and the Local Group
of which the Milky way is a part, would all remain virtually
undisturbed as the rest of the universe recedes and disappears from
view. In this scenario, the
Local GroupLocal Group would ultimately suffer heat
death, just as was hypothesized for the flat, matter-dominated
universe before measurements of cosmic acceleration.
There are other, more speculative ideas about the future of the
universe. The phantom energy model of dark energy results in divergent
expansion, which would imply that the effective force of dark energy
continues growing until it dominates all other forces in the universe.
Under this scenario, dark energy would ultimately tear apart all
gravitationally bound structures, including galaxies and solar
systems, and eventually overcome the electrical and nuclear forces to
tear apart atoms themselves, ending the universe in a "Big Rip". It is
also possible the universe may never have an end and continue in its
present state forever[citation needed] (see The second thermodynamics
law as a law of disorder). On the other hand, dark energy might
dissipate with time or even become attractive. Such uncertainties
leave open the possibility that gravity might yet rule the day and
lead to a universe that contracts in on itself in a "Big Crunch",[79]
or that there may even be a dark energy cycle, which implies a cyclic
model of the universe in which every iteration (
Big BangBig Bang then
eventually a Big Crunch) takes about a trillion (1012) years.[80][81]
While none of these are supported by observations, they are not ruled
out.
In philosophy of science[edit]
In philosophy of science, dark energy is an example of an "auxiliary
hypothesis", an ad hoc postulate that is added to a theory in response
to observations that falsify it. It has been argued that the dark
energy hypothesis is a conventionalist hypothesis, that is, a
hypothesis that adds no empirical content and hence is unfalsifiable
in the sense defined by Karl Popper.[82]
See also[edit]